Rafa Benitez has defended his decision to take off Matt Ritchie – after fans booed his substitution.
Ritchie was taken off during this afternoon's FA Cup defeat to Watford at St James's Park.
Second-half goals from Andre Gray and Isaac Success gave the visitors a 2-0 win and a place in the fourth round of the competition.
And some supporters booed the 73rd-minute withdrawal of Ritchie, who was replaced by Christian Atsu.
Asked about the reaction, Benitez said: "I understand, but when you have players playing so many games you have to be careful to protect them sometimes."
United's manager also opted against sending on leading scorer Salomon Rondon, who was an unused substitute.
Benitez instead sent on defender Fabian Schar for the last 10 minutes.
"We didn't create too many chances and that was not because of the strikers," said Benitez, who fielded Joselu up front.
On the result, Benitez said: "I'm disappointed, obviously. I think we could have won this game, but we didn't do it."